Na₃V₂(PO₄)₃@Carbon Nanofibers: High Mass Loading Electrode Approaching Practical Sodium Secondary Batteries Utilizing Ionic Liquid Electrolytes

Practical sodium secondary batteries require high power, high energy density, and long cyclability. The NASICON-type Na3V2(PO4)3 (NVP) is often investigated as a positive electrode material due to its high operation voltage, structural stability, and high Na+ ion conductivity. To overcome its low electronic conductivity, NVP requires carbon-coating or the addition of conductive materials for practical use. In this study, carbon nanofibers (CNFs) are incorporated as a conductive material along with glucose for carbon coating and fixing CNF frames to NVP particles. Uniform NVP composite and CNFs network (NVPC@CNFs) are obtained by a combination of sonication and the sol–gel method. Electrochemical measurements using a high mass loading electrode around ∼8.5 mg-active material cm–2 and Na[FSA]-[C2C1im][FSA] (C2C1im = 1-ethyl-3-methylimidazolium, FSA = bis(fluorosulfonyl)amide) ionic liquid electrolyte suggest safe operations of sodium secondary batteries up to intermediate temperatures (∼373 K). The rate performance further improved by using the NVPC@CNFs compared to NVPC and exhibited a high rate capability (at high geometric current density) of 51.1 mAh g–1 at 10C (10.0 mA cm–2) at 298 K and 82.3 mAh g–1 at 100C (100 mA cm–2) at 363 K (1C = 118 mA g–1, 1.00 mA cm–2). Furthermore, this material with an ionic liquid electrolyte exhibited superior Coulombic efficiencies over 3000 cycles of 99.9%. Electrochemical measurements (electrical impedance spectroscopy, charge–discharge test, cycle test, and rate performance test) clarify the electrochemical characteristics of this material.